In a large bowl, combine 1 cup chopped fresh spinach, 1 teaspoon minced garlic, ½ cup grated yellow onion, 1 teaspoon dried oregano, ½ teaspoon kosher salt, ¼ teaspoon ground black pepper, 6 ounces crumbled feta , and 1 pound ground turkey until just combined.
Divide the turkey mixture into 4 equal portions and shape each one into a 1-inch thick patty.
Heat a heavy skillet or cast-iron pan over medium-high heat. Add the patties to the hot pan and cook for 4-5 minutes on each side, only flipping them once.
While the patties cook, combine ½ cup plain Greek yogurt, 2 tablespoons fresh lemon juice, ¼ teaspoon kosher salt, and 2 pinches ground black pepper in a small bowl.
Lightly toast 4 hamburger buns, if desired. Add the sauce to the buns, followed by the green leaf lettuce, sliced tomato, and turkey patties. Serve warm.